Internal combustion engines, in particular diesel engines, need an additional heat source for preheating the gas mixture, the intake air, or the combustion chamber for starting and warm-up, in particular at low temperatures.
In passenger cars and sometimes in trucks, sheathed-element glow plugs may be provided and used for this purpose. At least part of the heating element of such a sheathed-element glow plug protrudes into the combustion chamber of the engine and preheats the combustion chamber before the actual engine start and in cold run. In addition, the fuel may ignite on the hot surface of the sheathed-element glow plug.
Sheathed-element glow plugs may be made of a plug housing having a glow tube or a tubular heating element situated thereon. A glow filament, embedded in a compacted magnesium oxide powder, is situated in the tubular heating element made of a material resistant to corrosion by hot gases. The glow filament is usually made of two resistors connected in series: a control filament and a heating filament. The heating filament has an electrical resistance which is almost independent of the temperature, and the control filament is made of a material having a positive temperature coefficient (PTC). The heating filament is welded into the cap of the glow tube on the ground side for contacting. The control filament is contacted to the terminal stud which is connected to the vehicle electrical system.
When a voltage is applied to the glow plug, most of the electric power is converted into heat in the heating filament, and the temperature at the tip of the plug increases steeply. The temperature of the control filament, and thus also its resistance, increase with a time delay. Current consumption and thus the total power consumption of the sheathed-element glow plug are reduced and the temperature approaches a state of equilibrium. Currently sheathed-element glow plugs designed for 12 V to 24 V vehicle electrical systems are commercially available.